This invention relates to magnetic resonance (MR) techniques. More specifically, this invention relates to the automatic adjustment of the RF transmitter and receiver to the optimal Larmor frequency. The invention is particularly applicable to magnetic resonance imaging, but is not limited thereto.
The magnetic resonance phenomenon has been utilized in the past in high resolution magnetic resonance spectroscopy instruments by structural chemists to analyze the structure of chemical compositions. More recently, MR has been developed as a medical diagnostic modality having applications in imaging the anatomy, as well as in performing in vivo, noninvasive spectroscopic analysis. As is now well known, the MR phenomenon can be excited within a sample object, such as a human patient, positioned in a homogeneous polarizing magnetic field, B.sub.o, by irradiating the object with radio frequency (RF) energy at the Larmor frequency. In medical diagnostic applications, this is typically accomplished by positioning the patient to be examined in the field of an RF coil having a cylindrical geometry, and energizing the RF coil with an RF power amplifier. Upon cessation of the RF excitation, the same or a different RF coil is used to detect the MR signals, frequently in the form of spin echoes, emanating from the patient lying within the field of the RF coil. In the course of a complete MR scan, a plurality of MR signals are typically observed. The MR signals are used to derive MR imaging or spectroscopic information about the patient being imaged or studied.
Before the commencement of each MR scan, it is common practice to adjust the frequency of the RF transmitter and receiver to insure that the excitation field is at the optimal Larmor frequency. This is necessary to produce the desired image contrast effects in certain MR measurements and to insure the accuracy of slice selection location. In a human subject, for example, the MR signal is produced primarily by the protons in water and fat molecules. The Larmor frequency of the protons in these two substances is slightly different and the Larmor frequency of both will vary slightly from patient to patient and at different locations within a patient due to inhomogeneities. In prior MR scanners, it is common practice to perform a calibration sequence in which an MR sequence is first executed and the MR signal is processed to produce on a CRT screen a picture of signal amplitude versus RF frequency. The operator then examines this picture and manually adjusts the frequency of the RF receiver to a desired value. For example, the displayed MR signal may show two peaks, one at the Larmor frequency for fat protons and one at the Larmor frequency for water protons. The operator may choose either frequency, or a frequency therebetween, depending on the particular MR measurement to be conducted.